As shown in FIG. 1 and FIG. 2 (which is a partial enlarged view of FIG. 1; it is to be noted that in both FIGS. 1 and 2, the blade portion is, for ease of understanding, depicted extremely large as compared with the cylinder), in gravure (intaglio) printing, a cylinder (1) having on its peripheral surface a large number of minute recesses (not shown) called cells corresponding to an image is used. A doctor blade (2) formed of steel or stainless steel is pressed against the peripheral surface of this cylinder with a fixed pressure, thereby scraping off ink (3) adhering to a non-image portion of the plate surface. This doctor blade serves to completely remove the ink of the non-image portion and to leave a predetermined amount of ink on the image portion, so that the contact pressure of the cylinder and the doctor blade has to be always maintained at a fixed level, and the edge part of the blade is required to have wear resistance. Thus, a doctor blade which has undergone surface treatment such as plating (4) is generally used.
For example, there are proposed: (1) in JP 4-296556 A, a doctor blade whose edge portion has a surface formed of an ink-repellent material (metal plating containing polymer particles);    (2) in JP 2001-80230 A, a backing-of-a-spatula preventing doctor blade whose surface has undergone fluorine-containing treatment (e.g., a metal plating of eutectoid 4-fluorinated ethylene resin particles);    (3) in JP 2000-507523 A, a doctor blade whose surface is coated with a polymer having a poor surface energy of 10 to 60 mN/m; and    (4) in JP 3-64595 A, a surface treated doctor blade consisting of a first layer of a nickel-based alloy and an upper layer of a chromium plating and superior in rust resistance and wear resistance.    (5) In JP 2952333 B, there is proposed a method of manufacturing a doctor blade having a double-layered plating consisting of a first layer of Ni plating and an upper layer of Ni plating containing ceramic powder.    (6) In JP 2001-1664 A, there is proposed a doctor blade in which the surface of the blade core metal is coated with a primer plating coating film and a diamond-like carbon coating film formed thereon, the primer plating being harder than the core metal and softer than the diamond-like carbon coating film.
However, these prior-art techniques have the following problems:
In the technique as proposed in (1), the surface of the blade edge is ink repellent, and doctoring is performed without extracting the ink from the cells of the plate, whereby it is possible to fill the cells with ink in a satisfactory manner, and even in precision printing, the configurations of the pixels can be reproduced in a satisfactory manner. However, since the surface treatment is effected with a single layer of metal containing ink repellent polymer particles, the wear resistance is insufficient, and as continuous printing is conducted, the wear of the blade edge progresses rapidly, so that it is necessary to frequently replace the blade, resulting in reduction in printing efficiency.
In the technique as proposed in (2), coating is effected on a blade solely by metal plating in a single layer containing 4-fluorinated ethylene resin particles, whereby it is possible to effectively prevent a printing failure called “backing of a spatula”, in which during continuous printing, coating liquid is accumulated and grows on the back side of the blade edge (back side with respect to the direction to which the cylinder rotates (R side in FIG. 2)), and the accumulated liquid drops are irregularly transferred to the original form to swell in dots or streaks. However, with the metal plating using eutectoid 4-fluorinated ethylene resin particles alone, it is impossible to obtain a sufficient degree of wear resistance since the plating is relatively soft, and the wear of the blade edge progresses rapidly with continuous printing, so that it is necessary to frequently replace the blade, resulting in a deterioration in printing efficiency. Further, with this single layer plating alone, a failure in scraping ink off the cylinder is likely to occur, and a printing failure (fog or the like) assumed to be attributable to the scraping failure occurs in a relatively early stage of continuous printing.
In the technique as proposed in (3), the blade surface is coated with a polymer with a poor surface energy to thereby prevent ink deposit when a high viscosity ink is used. However, since the substance with which the blade is coated is a polymer, the wear resistance of the blade is not improved at all, and it is necessary to frequently replace the blade as a result of the wear of the blade edge, resulting in a low production efficiency. Further, a failure in scraping chemical liquid off the cylinder is likely to occur, and a printing failure (fog or the like) occurs at a relatively early stage of printing.
In the techniques as proposed in (4) and (5), double-layered plating is effected to thereby enhance adhesion of the upper plating layer, and the wear resistance of the blade edge is improved by chromium plating forming the outermost layer or ceramic-containing-nickel plating. Although the techniques are effective in mitigating the wear of the blade, since the plating coating film forming the outermost layer is hard, there is a problem in that a printing failure due to acceleration of the wear of the cylinder itself, damage of the cylinder, peeling of the plating coating film or the like is likely to occur.
In the technique as proposed in (6), two-layer treatment is adopted using a ceramic composite nickel coating film and a diamond-like carbon coating film, thereby preventing plate fog due to a failure in scraping ink off when using aqueous ink. However, this technique has a problem in that adhesion between the diamond-like carbon coating film and the ceramic composite nickel coating film is insufficient. As the blade edge end is worn during continuous printing, the upper layer is likely to be peeled, and the resultant peeled powder is mixed with the ink, so that printing failure is likely to occur. Further, there is a problem in that the production efficiency when providing the diamond-like carbon coating film is rather low, and the provision requires a special apparatus such as a plasma deposition apparatus, with the result that a production cost itself is high, or the like.
Further, it has recently become more difficult to meet the user needs for accuracy. There is a requirement for a more accurate reproducibility for image configuration, and minute image defects such as bleeding and blurring which were overlooked in the past matter much more nowadays. This problem cannot be solved by the prior-art techniques.
It is accordingly an object of the present invention to provide a surface treated doctor blade which solves the above problems in the prior art and which helps to improve the wear resistance of the blade edge end and suppresses occurrence of printing failure during continuous printing.
On the other hand, in the doctor blades according to the prior-art techniques (1) to (6), the blade edge end is completely covered with plating (like the embodiment of the present invention as shown by FIG. 3(c)). Thus, when printing is performed immediately after replacing the blade with a new blade, a printing failure such as streaking or fog is generated due to defective contact between the cylinder and the blade edge end. In view of this, it is general practice to conduct a running-in for 30 to 60 minutes in order for the blade to obtain a good fit with the cylinder (conformability) which the blade contacts with, and then perform actual printing. As a result, there are problems in that a time loss corresponding to the running-in is involved, which means a very poor printing efficiency, and moreover, the cylinder can be damaged during the running-in, or partial wear can occur on the blade (hereinafter, such problems will be collectively referred to as “conformability of the blade”).
Thus, another object of the present invention is to provide a surface treated doctor blade improved in conformability between cylinder and blade edge end to thereby reduce running-in time, superior in wear resistance of the blade edge end, which can suppress occurrence of printing failure during continuous printing.